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Spondylolysis was first described in the medical literature in the mid-1800s and is comprised of the words 'spondylo' (vertebra) and 'lysis' (destruction). It involves the degeneration or defective development in the pars interarticularis - the thin segment of bone that connects the upper and lower facet joints of the vertebrae in the spinal column - resulting in the potential for slippage of one vertebra over another. The term spondylolisthesis refers to the forward slipping of one vertebra over another.

In combination with other factors, spondylolysis, also called isthmic spondylolisthesis, may permit forward slippage of one vertebra on the one below, resulting in spondylolisthesis. This may occur at any level of the spinal column but occurs most commonly at the lowest lumbar vertebra (L5) causing it to slip on the first sacral vertebra (S1).

Four to six percent of the general population may have spondylolysis. In the majority of young children vertebral development is normal but around the age of eight, some children experience abnormal growth and development of this particular area of the bone. The bony defect is virtually nonexistent among newborns, but exists in 5% of 6 year olds and approximately 6% of adults.

Most children and adults with spondylolysis are usually asymptomatic and they may discover its existence only incidentally when they seek medical advice for back pain that may not be related to the condition. Although spondylolysis develops in childhood, it remains unclear why for some people no symptoms ever appear and for others they may appear many years later. Some studies suggest that only 20% of adult patients with spondylolysis were symptomatic before the age of 20.

Spondylolysis is more common in persons who participate is sports such as diving, weight lifting, wrestling, and gymnastics - activities that require repetitive hyperextension of the lumbar portion of the spine. But repetitive stress over time, or an acute extension injury to the lumbar spine, may also cause the condition in susceptible adults.

The Meyerding's Grading System is most commonly used to measure the degree of slippage:

• Grade I = 1% to 25%
• Grade II = 26% to 50%
• Grade III = 51% to 75%
• Grade IV = 76% to 100%
• Grade V is called spondyloptosis and occurs when the L5 vertebra completely slides over the top of the sacrum.

Grade III and above is considered to be high-grade slippage. Slip progression after skeletal maturity is usually related to disk degeneration at the slip level. This is likely to develop during the fourth and fifth decades of life. The magnitude of symptoms is usually, but not necessarily, related to the grade of slippage.

Adult onset slip progression can cause severe incapacitating back and leg pain. Until recently, it has been thought that slip progression is rare in adults but that assumption is now being debated. Adult spondylolisthesis is frequently accompanied by disk degeneration at the slip level. As the disk loses it structural and functional integrity, the lumbosacral junction (L5-S1) becomes unstable and the slip progresses.

Slip progression usually occurs after the third decade of life and is seen in approximately 20-30% of adults with isthmic spondylolisthesis. It is associated with signs of mechanical instability and spinal stenosis resulting in significant low back pain and possibly radicular (radiating) pain in the back of the legs. This concurrent occurrence of disk degeneration and adult slip progression explains how asymptomatic spondylolysis or spondylolisthesis present for at least two or three decades may become symptomatic.